Weighing In Motion is defined as the process of measuring the dynamic tire forces of a moving road vehicle and estimating the gross-vehicle weight and the portion of that weight carried by each wheel, axle, or axle group of a corresponding static vehicle.
A WIM system implements the process of weighing a moving road vehicle by first measuring the dynamic (varying with time), vertically-downward component of the tire force from each wheel on the vehicle as the vehicle passes on a smooth road surface over specially-designed sensors. Then, the characteristics of these measured tire forces—along with other measured or calculated parameters such as speed and longitudinal position of the vehicle in the traffic lane—are used to estimate the gross-vehicle weight and the portion of that weight carried by each wheel, axle, and axle group of a corresponding static vehicle.
WIM was invented in the US (Texas) in the 50s by Pr Clyde Lee. Originally it was mainly used to collect large sample of axle and vehicle loads for pavement design purposes and the ASTHOO pavement design code. Bending plates were among the first sensors used for weighing. From the 70s, new technologies of WIM sensors were developed in Europe, such as wire, strip and bar sensors, capacitive, piezo-electric (ceramic, then polymer andthen quarz), and finally fiber optics. Bridge WIM was also introduced in the US in the late 70s and then developed in Europe in the 90s.
In the 70s and 80s, WIM data were used for bridge design code calibration (Ontario, Eurocode, etc.) and for bridge assessment, mainly in fatigue, but also to estimate the extreme loads and load effects. WIM data were also used for traffic monitoring and statistics on road freight transport.
In the 90s, the first WIM standard (ASTM-1318) was published in North America, and the COST323 action provided European specifications of WIM (pre-standard) as well as pan-European tests of WIM systems. The European research project WAVE (Weighing-in-motion Axles and Vehicles for Europe) and other initiatives delivered improved technologies and new methodologies of WIM.
In the early 2000s, the accuracy of WIM systems were significantly improved and they were used more and more for overload screening and preselection (enforcement). Now a new challenge is to adapt the WIM technology and type approval procedures for direct enforcement.
Depending on the WIM location and velocity, low and high speed WIM are distinguished:
- Low speed weigh-in-motion (LS-WIM): the weighing is done on a dedicated area, mostly outside the traffic lane, on a flat and smooth platform (generally in concrete), longer than 30 m, and at a velocity below 5 to 10 km/h, in order to eliminate the dynamic effects of the vehicle and to assume that the tire impact forces are equal to the wheel static loads. LS-WIM uses mainly load cell scales and is legally approved for direct enforcement or trade.
- High speed weigh-in-motion (HS-WIM): the weighing is done in the traffic lane at current speed (i.e. 60 to 90 or 100 km/h), without slowing down or stopping the vehicles. The measurements are affected by the vehicle dynamics (vertical accelerations), and may significantly differ from the static weights on uneven pavement surfaces.